1. Field of the Invention
The present invention relates to an amplifying circuit, a noise reducing apparatus, and a power converting apparatus, and particularly relates to a technique enabling to miniaturize them.
2. Description of the Related Art
Power converting apparatuses such as an inverter that supplies power to a motor, a switching regulator that supplies voltage to a computer and the like convert power sent from a predetermined power source into power of a predetermined voltage to supply to a load. In such the power converting apparatuses, since switching element is turned on/off to perform power conversion, switching noise is generated by switching the switching element. For this reason, in such the power converting apparatuses, a noise filter is conventionally provided at an input side to reduce noise. Since the frequency of switching noise is greatly high, a noise filter with large attenuation characteristics in a board band is demanded.
While, in the circuit, an electrostatic capacity containing a stray capacity between the circuit and ground is present, and noise caused by switching the switching element becomes a leakage current with high frequency and flows into a ground line. When the leakage current flows into the ground line, a voltage level of a frame (housing) of the power converting apparatus varies. Particularly, when a motor with a high power capacity is connected to the power converting apparatus via the inverter, the stray capacity between the circuit and the ground increases and the leakage current also increases accordingly. The increase in the leakage current will trip a ground-fault circuit interrupter and cause interference with peripheral electronic devices.
For this reason, a method for supplying a compensation current to a ground line in a direction that cancels a leakage current is used as a method for reducing such noise. This method will be explained based on FIGS. 38A and 38B. In FIGS. 38A and 38B, capacitors C201 and C202 correspond to a load electrostatic capacity and a capacitor for common mode noise, respectively. A diode D201 corresponds to a rectification circuit, and a switch SW corresponds to a switching element. Moreover, leakage currents Is1 and Is2 indicate a leakage current flowing from an alternating current source ACV by switching the switch SW and a leakage current propagating in the power converting apparatus, respectively.
Two methods can be considered as a noise reducing method. A first method is one in which a leakage current Is1 is detected and the detected current is amplified by an amplifier AMP, and an amplified compensation current Ir is supplied to a ground line in a direction that cancels the leakage current Is1 via the capacitor C202 as illustrated in FIG. 38A. According to this method, a zero phase current transformer 201 is provided closer to the alternating current source ACV side than an injection point a of the compensation current Ir so as to detect the leakage current Is1.
In the first method, the following equation (1) is established:{A1·(is1−ir)}−is1≈0  (1)(where A1 is an amplification factor of the amplifier AMP when the first method is used, is1 is a current value of the leakage current Is1, and ir is a current value of the compensation current Ir).
Accordingly, the current value ir of the compensation current Ir can be expressed by the following equation (2).ir={1−(1/A1)}·is1  (2)
A second method is one in which a leakage current Is2 is detected and a compensation current Ir is supplied to the ground line based on the detected current in the same way as the first method as illustrated in FIG. 38B. According to this method, a zero phase current transformer 201 is provided closer to the switch SW side than the injection point a of the compensation current Ir so as to detect the leakage current Is2.
When the second method is used, the following equation (3) is established:is1−(A2·ir)=0  (3)(where A2 is an amplification factor of the amplifier AMP when the second method is used).
Accordingly, the current value ir of the compensation current Ir can be expressed by the following equation (4).ir=(1/A1)·is1  (4)
As shown in the equation (4), in order to cancel the leakage current Is1 by the compensation current Ir using the second method, the amplification factor A2 of the amplifier AMP must be correctly set to 1. In the conventional power converting apparatus, since there is difficulty in that the amplification factor A2 of the amplifier AMP is correctly set to 1, the first method is generally used (Unexamined Japanese Patent Publication 9-266677).
However, when the first method is used, the amplification factor A1 of the amplifier AMP must be increased in order to cancel the leakage current Is1 by the compensation current Ir as shown in the equation (2). For this reason, phase compensation must be correctly performed, and a disadvantage that the amplifier AMP does not easily oscillate is caused.
On the other hand, when the second method is used, the amplification factor A2 of the amplifier AMP does not have to be increased. However, when the second method is used, the amplification factor A2 of the amplifier AMP must be correctly set to 1.
Morever, it is necessary to cause a secondary winding of the zero phase current transformer 201 to generate an emitter-base voltage of a transistor provided in the amplifier AMP, resulting in an increase in the number of the secondary windings.
Furthermore, in either method, a CT (current transformer) for detection is used as the zero phase current transformer 201 to detect a leakage current flowing into a power line as a current difference. Then, a compensation current is directly detected by the CT for detection and supplied to the power line from a noise compensation current supply current. In this case, in order to operate the amplifier AMP, it is necessary to cause the secondary winding for the CT for detection to generate E-B voltage (0.6 V) of the transistor provided in the amplifier AMP, resulting in an increase in the number of the secondary windings. Particularly, when the second method is used, a turn ratio between the primary winding and secondary winding of the zero phase current transformer 201 must be 1:1.
Moreover, since a large current flows into the power line, a diameter of the primary winding of the zero phase current transformer 201 increases. For this reason, there is a problem in which the zero phase current transformer 201 is increased in size, making it difficult to use the zero phase current transformer 201 in the large current.
Moreover, there was a problem in which a neutral point voltage of the transistor did not reach a neutral point of a supply voltage, so that a noise control range became narrow. Furthermore, since these noise reducing apparatuses needed a greatly large noise filter, they were increased in size and this was one of causes in an increase in cost.
In order to adjust the amplification factor in the aforementioned methods, the winding number of the zero phase current transformer 201 must be increased to perform adjustment and this will cause an increase in size of the zero phase current transformer 201. Moreover, the amplifier AMP with higher accuracy is required, resulting in an increase in cost.